Exploring the Influence of Inter‐ and Intra‐crystal Diversity of Surface Barriers in Zeolites on Mass Transport by Using Super‐Resolution Microimaging of Time‐Resolved Guest Profiles

Abstract

The applications of nanoporous crystalline materials are closely related to the mass transfer of guest molecules. However, the fundamental knowledge of mass transfer, and in particular the surface barriers controlled by the permeation of guest molecules through the external surfaces of materials, is still incomplete. The diversity of surface permeability at the single‐crystal level, caused by the varying origins of surface transport resistance, hinders the rational materials design and needs better understanding. Herein, we probe the molecular transport in single zeolite crystals with fluorescent 4‐(4‐diethylaminostyryl‐1‐methylpyridinium iodide) (DAMPI) using super‐resolution structured illumination microscopy (SIM). It showed that both the inter‐ and intra‐crystal diversity of surface barriers could be monitored by detecting the diffusion behaviors on the center and surface planes in single crystals. This adds a new perspective for studying the origins of the surface barriers as well as the molecular transport mechanisms in nanoporous materials.

Surface barriers to mass transfer in nanoporous materials for catalysis and separations

“…They discovered a noticeable jump in boundary concentration compared with the equilibrium value, thus indicating the influence of external surface barriers. 20,29 In comparison, the equilibrium value is quickly achieved after a quite short time interval in the absence of notable external surface barriers, as shown in Fig. 7(B).…”

“…Recently, single-molecule fluorescence imaging has been developed as a technique to reveal spatiotemporal gradients of product molecules in zeolite crystals. 26,29…”

“…Recently, single-molecule fluorescence imaging has been developed as a technique to reveal spatiotemporal gradients of product molecules in zeolite crystals. 26,29 The quantification of surface barriers used to be complicated due to the requirement of both apparent diffusivities acquired from macroscopic measurements, as well as intracrystalline diffusivity data derived from microscopic measurements or molecular simulation. Recently, Gao et al 17 put forward a dual-resistance model (DRM) to decouple the surface barriers from intracrystalline diffusivity in nanoporous materials without the need for any preliminary data.…”

“…18,19 To provide direct evidence of surface barriers, concentration profiles of guest molecules have been recorded using interference microscopy (IFM) and infrared microscopy (IRM) by Ka ¨rger and co-workers. [20][21][22][23][24][25] Super-resolution fluorescence microscopy (FM) 26,27 and structured illumination microscopy (SIM) [28][29][30] have also been successfully extended to the characterization of mass transfer and heterogeneous catalysis. The adjustable confocal plane enables the observation of local density distributions of guest molecules, which enter and move through a specific zeolite crystal.…”

Cooperative External Acidity and Surface Barriers of HZSM-5 in the Coupling Reaction of CH3Cl and CO to Aromatics

“…Thus, significant studies have been dedicated to the crystal surface microenvironment, such as external acidity or surface barriers, through surface modification methods. On the one hand, previous studies, such as chemical liquid/vapor deposition, phosphorus modification, and epitaxial growth, focused on the effect of external acidity without considering the surface barriers resulting from surface passivation and concluded that reducing external acidity can prevent side reactions and external coke deposition. − On the other hand, modulating surface barriers allows for the regulation of the reactant and product mass transfer through the crystal surface, hence regulating the catalytic efficacy regardless of the changes of external acidity. − However, it can be concluded easily that even the trivial changes of acidity should be resulted in changing surface barriers and vice versa. Since the complex interrelations between external acidity and surface barriers, independently investigating the effects of surface barriers and external acidity on the production of aromatics over zeolite continues to be an open challenge.…”